This invention relates in general to devices for actuating trailer electric wheel brakes and in particular to enhancements for trailer electronic wheel brake controllers.
Towed vehicles, such as recreational and utility trailers which are towed by automobiles and small trucks, are commonly provided with electric wheel brakes. The electric wheel brakes generally include a pair of brake shoes which, when actuated, frictionally engage a brake drum. An electromagnet is mounted on one end of a lever to actuate the brake shoes. When an electric current is applied to the electromagnet, the electromagnet is drawn against the rotating brake drum which pivots the lever to actuate the brakes. Typically, the braking force produced by the brake shoes is proportional to the electric current applied to the electromagnet. This electric current can be relatively large. For example, the electric wheel brakes on a two wheeled trailer can draw six amperes of current when actuated and the electric wheel brakes on a four wheeled trailer can draw 12 amperes of current.
Automotive industry standards require that electrically-actuated vehicle wheel brakes be driven against the ground potential of the vehicle power supply. Accordingly, one end of each of the towed vehicle wheel brake electromagnets is electrically connected to the towed vehicle ground and the towed vehicle ground is electrically connected to the towing vehicle ground. The other end of each of the brake electromagnets is electrically connected through either an electric wheel brake actuator or an electric wheel brake controller to the towing vehicle power supply.
Generally, electric wheel brake actuators are manually operated devices which control the magnitude of electric current supplied to the towed vehicle wheel brakes. Various electric brake controllers for towed vehicle electric brakes are known in the art. For example, a variable resistor, such as a rheostat, can be connected between the towing vehicle power supply and the brake electromagnets. Such an actuator is disclosed in U.S. Pat. No. 3,740,691. The towing vehicle operator manually adjusts the variable resistor setting to vary the amount of current supplied to the brake electromagnets and thereby control the amount of braking force developed by the towed vehicle wheel brakes.
It is also known to include an integrating circuit in an electric wheel brake actuator. When the towing vehicle brakes are applied, a signal is sent to the integrating circuit. The integrating circuit generates a continually increasing voltage which is applied to the electric wheel brakes. The longer the towing vehicle brakes are applied, the more brake torque is generated by the actuator. A manually adjustable resistor typically controls the rate of integration. On such actuator is disclosed in U.S. Pat. No. 3,738,710.
Also known in the art are more sophisticated electric wheel brake controllers which include electronic circuitry to automatically supply current to the towed vehicle brake electromagnets which is proportional to the towing vehicle deceleration when the towing vehicle brakes are applied. Such electronic wheel brake controllers typically include a sensing unit which generates a brake control signal corresponding to the desired braking effort. For example, the sensing unit can include a pendulum which is displaced from a rest position when the towing vehicle decelerates and an electronic circuit which generates a brake control signal which is proportional to the pendulum displacement. One such unit is disclosed in U.S. Pat. No. 4,721,344. Alternately, the hydraulic pressure in the towing vehicle""s braking system or the pressure applied by the vehicle operator""s foot to the towing vehicle""s brake pedal can be sensed to generate the brake control signal. An example of a controller which senses the towing vehicle brake pressure to generate the brake control signal is disclosed in U.S. Pat. No. 4,398,252.
Known electronic wheel brake controllers also usually include an analog pulse width modulator. The input of the pulse width modulator is electrically connected to the sensing unit and receives the brake control signal therefrom. The pulse width modulator is responsive to the brake control signal for generating an output signal comprising a fixed frequency pulse train. The pulse width modulator varies the duty cycle of the pulse train in direct proportion to the magnitude of the brake control signal. Thus, the duty cycle of the pulse train corresponds to the amount of braking effort desired.
Electronic wheel brake controllers further include an output stage which is electrically connected to the output of the pulse width modulator. The output stage typically has one or more power transistors which are connected between the towing vehicle power supply and the towed vehicle brake electromagnets. The power transistors, which are usually Field Effect Transistors (FET""s), function as an electronic switch for supplying electric current to the towed vehicle brakes. The output stage may also include a drive circuit which electrically couples the output of the pulse width modulator to the gates of the FET""s.
The output stage is responsive to the pulse width modulator output signal to switch the power transistors between conducting, or xe2x80x9conxe2x80x9d, and non-conducting, or xe2x80x9coffxe2x80x9d, states. As the output transistors are switched between their on and off states in response to the modulator output signal, the brake current is divided into a series of pulses. The power supplied to the towed vehicle brakes and the resulting level of brake application are directly proportional to the duty cycle of the modulator generated output signal.
This invention relates to enhancements for trailer electronic wheel brake controllers.
As explained above, electronic wheel brake controllers energize the towed vehicle brakes upon detection of the towing vehicle deceleration when the towing vehicle brakes are applied. However, other conditions, such as travel over a rough road surface may cause the pendulum in a deceleration sensor to be displaced and thereby generate a false brake control signal. Accordingly, it would be desirable to reduce the sensitivity of the sensing unit to filter out such spurious inputs.
Additionally, modern towing vehicles are equipped with capacity alternators that can supply large amounts of current. Furthermore, the voltage output of such alternators tends to fluctuate with load conditions. Accordingly, the current supplied to the trailer brakes could, under certain conditions become excessive. However, use of a fuse or circuit breaker to protect the trailer brake coils is not desirable since it would have to be replaced or reset after every occurrence of excessive current. Accordingly, it would also be desirable to provide a means for limiting the current supplied from the wheel brake controller to the trailer brakes.
The present invention contemplates a device for controlling the electric current supplied to at least one electric wheel brake which includes a brake control signal generator which is adapted to be connected to a vehicle stop light switch. The brake control signal generator is operative to generate a brake control signal that is a function of the towing vehicle deceleration. The device also includes a brake control signal amplifier that is connected to the brake control signal generator. A damping capacitor is coupled to the brake control signal amplifier to reduce sensitivity of the brake control signal generator. The brake control signal amplifier has an output that is connected to an output signal generator. The output signal generator has an output terminal and is responsive to an amplified brake control signal to generate an output signal at the output terminal which is a function of the brake control signal. The device further includes an electric current controller which is adapted to be connected between a vehicle power supply and the controlled electric wheel brake. The current controller also is coupled to the output terminal of the output signal generator and is responsive to the output signal to control the electric current supplied to the controlled wheel brake as a function of the output signal.
The invention further contemplates a current limiting circuit coupled to the current controller and the output signal generator. The current limiting circuit is operable to modify the output signal to progressively reduce the current supplied to the controlled wheel brake upon the current exceeding a first predetermined threshold. Additionally, the current limiting circuit is operative to disable the output signal generator upon the current being supplied to the controlled wheel brake exceeding a second predetermined threshold which is greater than the first predetermined threshold.